A Stream of binary ones at I Mbps( figure 5. 5) 5 bits=5 us posno NRZI I bit 1 signal element= 1 us n Manchester I bit= I signal element= 1 0.5 Hs igure 5.5 A Stream of Binary Ones at 1 Mbps
11 A Stream of binary ones at 1 Mbps (Figure 5.5)
3. Scrambling Techniques(扰码技术) Although the biphase techniques have achieved widespread use in local area network applications at relatively high data rate(up to 1OMbps), they have not been widely used in long-distance applications. The principal reason for this is they require a high signaling rate relative to the date rate. This sort of nefficiency is more costly in a long-distance application Another approach is to make use of some sort of scrambling scheme. The idea behind this approach is simple: Sequences that would result in a constant voltage evel on the line are replaced by filling sequences that will provide sufficient transmissions for the receiver's clock to maintain synchronization. The filling sequence must be recognized by the receiver and replaced with the original data sequence. The filling sequence is the same length as the original sequence, So there is no data rate increase. The design goals include: no DC component, no long sequences of zero-level line signals, no reduction in data rate, error- detection capability
12 3. Scrambling Techniques (扰码技术) Although the biphase techniques have achieved widespread use in local area network applications at relatively high data rate (up to 10Mbps), they have not been widely used in long-distance applications. The principal reason for this is they require a high signaling rate relative to the date rate. This sort of inefficiency is more costly in a long-distance application. Another approach is to make use of some sort of scrambling scheme. The idea behind this approach is simple: Sequences that would result in a constant voltage level on the line are replaced by filling sequences that will provide sufficient transmissions for the receiver’s clock to maintain synchronization. The filling sequence must be recognized by the receiver and replaced with the original data sequence. The filling sequence is the same length as the original sequence, so there is no data rate increase. The design goals include: no DC component, no long sequences of zero-level line signals, no reduction in data rate, errordetection capability
Two techniques are commonly used in long-distance transmission services; these are illustrated in Figure 5.6(E6) (1 A coding scheme that is commonly used in North America is known as bipolar with8- zeros substitution(B8ZS,双极性八位替换) The coding scheme based on a bipolar-AMI. We have seen that the drawback of the AMI code is that a long string of zeros may result in loss of synchronization. To overcome this problem the encoding is amended with the following rules ■ If an octet(八位组) of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+-0-+ If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000-+0+ (2) A coding scheme that is commonly used in Europe and Japan is known as the high- density bipolar-3 zeros(HDB3,双极性3零码)code. As before, it is based on the use of AMI encoding. In this case, the scheme replaces strings of four zeros with sequence containing one or two pulses
13 Two techniques are commonly used in long-distance transmission services; these are illustrated in Figure 5.6（E6）. （1）A coding scheme that is commonly used in North America is known as bipolar with 8-zeros substitution (B8ZS，双极性八位替换). The coding scheme is based on a bipolar-AMI. We have seen that the drawback of the AMI code is that a long string of zeros may result in loss of synchronization. To overcome this problem the encoding is amended with the following rules: ◼ If an octet（八位组）of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+-0-+. ◼ If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000-+0+-. （2）A coding scheme that is commonly used in Europe and Japan is known as the high-density bipolar-3 zeros (HDB3，双极性3零码) code. As before, it is based on the use of AMI encoding. In this case, the scheme replaces strings of four zeros with sequence containing one or two pulses
An Example slal0:°:°:0:°:0:°:0:1:1:0:0:0:0:0:1:0 Bipolar-AMI :n:m:o:o:o:vB:o:v:B: n:s:v bo:iiw B8ZS 正, 高点和* 列计出柳地:划 0:0:0:V:B:0:0 ::B:0:0:v:: HDB3 (odd number of Os since last B= valid bipolar signal V=Bipolar violation Figure 5.6 Encoding Rules for B8ZS and HDB3 ruts
14 An Example
In each case, the fourth zero is replaced with a code violation. In addition a rule is needed to ensure that successive violations are of alternate polarity so that no DC component is introduced. Thus, if the last violation was positive, this violation must be negative and vise versa 15
15 In each case, the fourth zero is replaced with a code violation. In addition, a rule is needed to ensure that successive violations are of alternate polarity so that no DC component is introduced. Thus, if the last violation was positive, this violation must be negative and vise versa